1. Field of the Invention
The present invention relates to a method of preventing the deterioration of the film quality of a transparent conductive film, to a semiconductor device and to a method of manufacturing a semiconductor device.
2. Description of the Related Art
A portable information processor is indispensable for performing electronic data processing (EDP) on an individual level, and a liquid crystal display unit is used as a man-machine interface required for utilizing EDP. It is desired that the image display unit produce a display of high density and good color capable of performing advanced information processing, image processing or the like, at a low cost.
An active matrix type liquid crystal display unit is constructed of switching elements are connected to a plurality of display units called picture elements, making a high grade display possible. A semiconductor element, such as a diode or a transistor, is used as the switching element.
The properties of the switching elements should be excellent because they are the primary factors for determining the number of gradation and the display capacity of the liquid crystal display unit. Further, since these switching elements are manufactured through a complicated process accompanied by fine processing. Simplifying the process for making switching elements, simplifies the process for making the entire liquid crystal display unit.
As a means for simplifying the manufacturing process for the switching element, it has been considered to form a new pattern on the pattern already formed on a substrate by self alignment. A technique exists for growing a thin film selectively on a conductive pattern as a process for forming a self aligned pattern.
Since it is possible, by selective growth, to form a layer selectively only on a thin film applied with patterning on a substrate, photolithography process becomes unnecessary. Further, since it is possible to grow still another thin film continuously without breaking vacuum after the selective forming process, contamination on interlayer interfaces is small, and the switching element properties are improved.
In the many selective growth techniques that have been developed, a chemical vapor deposition (CVD) method is utilized to achieve selective growth by the difference in chemical property of layer surfaces.
The CVD method is used frequently as a growth method for a semiconductor or a metal, but the growth rate is not fully controlled by the conditions that reflect the chemical disposition of the layer surface, selective deposition of a film does not always occur. However, it has become possible to obtain selective growth by reducing the deposition rate by diluting the growth gas with another gas, or by having the chemical disposition of the layer surface reflected by bringing etching and deposition close to an equilibrium state.
As a technique for the selective growth of silicon, there is, for example, the epitaxial lateral overgrowth (ELO) method disclosed in Japanese Patent Provisional Publication No. SH058-120595. That technique is a method of expanding the margin of a selective growth of silicon by repeating deposition and etching of layers alternately for a substrate having a region where silicon is grown and a region where silicon is difficult to be grown.
It has been disclosed in U.S. Pat. No. 5,242,530 that this technique is also applicable to a plasma CVD method widely used in the manufacture of a solar cell and an active matrix type liquid crystal display unit.
These techniques involve repeating etching and deposition alternately by utilizing a slight difference in the silicon growth rate produced by a difference between materials forming the layers. However, since the chemical state of the surface of a layer depends on the material constituting the layer, growth has to be performed very slowly in order to achieve selective silicon growth between materials having similar chemical dispositions.
When the rate of film formation is increased in order to shorten process time, growth becomes non-selective. Thus, there is the problem that the film forming process has to be performed for a long period of time by slowing down the rate of film formation in order to achieve selective growth.
It is necessary to increase the amount of time for etching in the overall time for processing to remove the silicon film deposited on unused parts in order to make selective growth uniform in an active matrix having a large area. As a result, the period of time for film deposition is reduced, the overall time for the selective growth process gets longer, and productivity declines.
Further, it depends on the material constituting the surface of the layer where the film is grown whether silicon is grown or not, the state of the layer surface is also changed by contamination and by natural oxidation. Thus, when the technique of selective growth is examined practically from the viewpoint of a manufacturing process, it has more or less lacked practicability.
For example, ITO is an electrically degenerated semiconductor and functions as an electrode, but resembles in a chemical disposition silicon oxide which is the oxide, such as quartz and glass, constituting the substrate. Since ITO is an oxide, there is little difference in the process when silicon is grown on ITO and on the substrate. Further, the reducing gas used when silicon is grown selectively on the surface of the transparent conductive film deteriorates the surface of the transparent conductive film.
Accordingly, it is difficult to perform selective growth on either one of two films having similar chemical dispositions. Practically, according to the examination of the present inventor, it has been found that the reproducibility of selective growth is poor, non-selective deposition of silicon films occurs frequently and the silicon film is deposited on the quartz substrate only.